1. Field of the Invention
The present invention relates generally to the field of disc drive storage, and more particularly to lubricants used in disc drives to improve tribological properties.
2. Description of the Related Art
Computer disc drives commonly use components made out of thin films to store information. Both the read-write element and the magnetic storage media of disc drives are typically made from thin films.
FIG. 1A is an illustration showing the layers of a conventional magnetic media structure including a substrate 105, a seed layer 109, a magnetic layer 113, a diamond like carbon (DLC) protective layer 117, and a lube layer 121. The initial layer of the media structure is the substrate 105, which is typically made of nickel-phosphorous plated aluminum or glass that has been textured. The seed layer 109, typically made of chromium, is a thin film that is deposited onto the substrate 105 creating an interface of intermixed substrate 105 layer molecules and seed layer 109 molecules between the two. The magnetic layer 113, typically made of a magnetic alloy containing cobalt (Co), platinum (Pt) and chromium (Cr), is a thin film deposited on top of the seed layer 109 creating a second interface of intermixed seed layer 109 molecules and magnetic layer 113 molecules between the two. The DLC protective layer 117, typically made of carbon and hydrogen, is a thin film that is deposited on top of the magnetic layer 113 creating a third interface of intermixed magnetic layer 113 molecules and DLC protective layer 117 molecules between the two. Finally the lube layer 121, which is a lubricant typically made of a polymer containing carbon (C) and fluorine (F) and oxygen (O), is deposited on top of the DLC protective layer 117 creating a fourth interface of intermixed DLC protective layer 117 molecules and lube layer 121 molecules.
The durability and reliability of recording media is achieved primarily by the application of the DLC protective layer 117 and the lube layer 121. The DLC protective layer 117 is typically an amorphous film called diamond like carbon (DLC), which contains carbon and hydrogen and exhibits properties between those of graphite and diamond. Thin layers of DLC are deposited on disks using conventional thin film deposition techniques such as ion beam deposition (IBD), plasma enhanced chemical vapor deposition (PECVD), magnetron sputtering, radio frequency sputtering or chemical vapor deposition (CVD). During the deposition process, adjusting sputtering gas mixtures of argon and hydrogen varies the concentrations of hydrogen found in the DLC. Since typical thicknesses of DLC protective layer 117 are less than 100 Angstroms, lube layer 121 is deposited on top of the DLC protective layer 117 for added protection, lubrication and enhanced disk drive reliability. Lube layer 121 further reduces wear of the disc due to contact with the magnetic head assembly.
An example of a lubricant used to lubricate hard drive disks is Moresco lubricant developed by Matsumura Oil Research Company, Inc, which can be used as a lube layer 121. Although Moresco lubricants are conventional lubricants used to lubricate disks in hard drive applications, there are both advantages and disadvantages with using these lubricants as will be discussed below.
Other typical lubricants that can be used as lube layer 121 include Perfluoropolyethers (PFPEs), which are long chain polymers composed of repeat units of small perfluorinated aliphatic oxides such as perfluoroethylene oxide or perfluoropropylene oxide. As is well known in the art, Moresco lubricant as well as PFPEs are used as lubricants because they provide excellent lubricity, wide liquid-phase temperature range, low vapor pressure, small temperature dependency of viscosity, high thermal stability, and low chemical reactivity. These lubricants also exhibit low surface tension, resistance to oxidation at high temperature, low toxicity, and moderately high solubility for oxygen. Several different PFPE polymers are available commercially, such as Fomblin Z (random copolymer of CF2 CF2 O and CF2 O units) and Y (random copolymer of CF(CF3)CF2 O and CF2 O) including Z-DOL and AM 2001 from Montedison, Demnum (a homopolymer of CF2 CF2 CF2 O) from Daikin, and Krytox (homopolymer of CF(CF3)CF2 O).
FIG. 1B is a flow chart showing the typical steps used in an in-situ vapor lubrication process that deposits PFPE lubricant over a carbon layer. The process begins with step 145 by transferring a partially complete media with substrate 105, seed layer 109, and magnetic layer 113 into a vacuum chamber. The transferring process typically involves moving a disk, after depositing a magnetic layer on it, into a carbon deposition chamber without taking it out of vacuum. In step 150 a magnetic stack is deposited onto the seedlayer. In step 155 a protective overcoat 117 consisting of an amorphous carbon is deposited over the partially complete media. Typically the amorphous carbon layer is diamond like carbon (DLC) that has been deposited by conventional sputter deposition techniques. Next in step 160, the amorphous carbon is coated with a lube layer 121 made of Moresco lubricant or PFPE using a dipping process or an in-situ vapor lubrication process. Finally, in step 165 the lubed magnetic media is transferred to the next manufacturing operation.
In step 160, the lube layer 121 can be applied using several processes including dipping the magnetic media, which has a protective layer 117, into a tank of lubricant fluid or vapor lubing the disk. If the lube layer 121 is applied with a dipping process, the magnetic media with protective layer 117 is first taken out of vacuum and dipped into a tank of lubricant fluid. If lube layer 121 is applied using a vapor luber, then step 160 can be done in the same or different apparatus as step 155 was done.
In the dipping process, lube layer 121 is typically applied evenly over the disc, as a thin film, by dipping the discs in a bath containing a mixture of a few percent of the lubricant in a solvent and gradually draining the mixture from the bath at a controlled rate. The solvent remaining on the disc evaporates and leaves behind a layer of lubricant less than 100 Angstroms. Recent advances have enabled the application of PFPE using an in-situ vapor deposition process that includes heating the PFPE with a heater in a vacuum lube process chamber. In this system, evaporation occurs in vacuum onto freshly deposited DLC protective layer 117 that has not been exposed to atmosphere, creating a thin uniform coating of PFPE lube layer 121.
There are however, problems with using Moresco and PFPE lubricants in media for modern disc drives applications. For example, as the thickness of the protective layers 117 and lube layers 121 are reduced, reliability problems arise. A more integrated protection structure is needed that will produce a more durable protective film without effecting thicknesses. Reliability of hard disks is heavily dependent upon the durability of the thin film media.
Lubrication additives, such as Bis(4-fluorophenoxy)-tetrakis(3-trifluoromethyl phenoxy) cyclotriphosphazene (X1P), have been used to improve tribological performance and corrosion resistance of thin film media. However, problems like phase separation, head smear, etc. have greatly hindered such applications. Although, various attempts to solve these problems have been made including using the Moresco lubricants developed by Matsumura Oil Research Company, Inc. which is linked directly to cyclotriphosphazene, satisfactory results have not been achieved. New lubricants such as the Moresco lubricants give better contact-stop-start testing results under harsh conditions than do the other lubricants. Although these new lubricants are an improvement over old lubricants, there are still many problems such as lubricant pick-up, head smear and high stiction which prohibit the use of hybrid lubricants.
Therefore what is needed is a system and method which overcomes these problems and makes it possible to hybrid lubricants process that results in a reliable final overcoat with desirable properties. Desirable properties include a resulting lubricant that do not have problems such as phase separation, lubricant pick-up, head smear, high stiction, etc.